The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, is neither expressly nor impliedly admitted as prior art against the present disclosure.
Hybrid vehicles generally have two power sources. The internal combustion engine is a first power source and the electric motor is a second power source. These two power sources are integrated in a hybrid powertrain which usually consists of a small gasoline engine to achieve better fuel economy, and an electric motor or motors to provide power assistance whenever necessary. For example, power assistance may be provided when the vehicle is at launch, going up hill, towing, and high-speed passing. Some advanced hybrid powertrains are also equipped with an electrically variable transmission (EVT) to provide continuous vehicle speed changes without the restriction from engine operations.
Other advantages of hybrid vehicles include an independently operated internal combustion engine that operates at high efficiency and low cost, the regenerative brakes to recover vehicle kinetic energy at braking, and the vehicle accessories powered by an independent electric motor at a constant speed to achieve higher mechanical and energy efficiencies.
In an advanced hybrid powertrain system, the electrically variable transmission may be equipped with a feedback control system to enhance the capability and performance of the hybrid system. Driveline smoothness will be improved using the feedback control systems. Active driveline damping control may be provided to define the damping torque from the electrical motors in the electrically variable transmission to minimize driveline rotational vibrations during transient conditions such as at the engine start and stop, transmission gear shifting, and driveline clutches engaging. However, the closed-loop feedback control can cause the hybrid powertrain system to be unstable or resonate due to some unexpected and unknown system characteristics. Therefore, there is a need for a method and system for monitoring the operational stability of the hybrid powertrains equipped with a closed-loop feedback control system.